The Watchers
There are several theories and legends pertaining to the
concept and origin of Fallen Angels. The , describes the
fall of a group of angelic renegades known as "the Watchers"
.

Shemhazai, an angel of high rank, led a sect
of angels in a descent to earth to instruct humans in
righteousness. Their mission lasted for a few centuries,
but soon the soldiers/missionaries/angels became
corrupted in their lusting after human females. After
lusting, the fallen angels instructed the women in magic
and conjuring, mated with them, and produced offspring,
who would later be referred to as the Nephilim. This
activity was apparently in direct contradiction to their
orders

Genesis 6:2
the sons of God saw that the daughters of men were beautiful, and
they married any of them they chose. [See ]

1 Enoch 6:1-7 " In those days, when
the children of man had multiplied, it happened that there were born
unto them handsome and beautiful daughters. And the angels, the
children of heaven , saw them and desired them; and they said to one
another, 'Come, let us choose wives for ourselves from among the
daughters of man and beget us children.' And Semyaz, being their
leader, said unto them,'I fear that perhaps you will not consent
that this deed should be done, and I alone will become (responsible)
for this great sin.' But they all responded to him, 'Let us all
swear an oath and bind everyone among us by a curse not to abandon
this suggestion but to do the deed.' Then they all swore together
and bound one another by (the curse) And they were altogether two
hundred;"

The fallen angels then went to Enoch to
intercede on their behalf with God. The remainder of the book describes
Enoch's visit to Heaven in the form of a vision, and his revelations.

Enoch also mentions angels called Grigori,
{Grigori is derived from the Greek word egrḗgoroi which
translates roughly to Watchers} the Grigori are similar to the Watchers.
Their leader is Satanail. In this account only three angels fall upon
the earth to mate with Humans and beget giants.

The Book of Enoch was suppressed in the
Early Years of Christianity, {there are varying reasons and or
excuses for this. Not the least of which being that Fallen Angels
had influenced in one way or another the compilation of the Bible,
not the scriptures themselves, but the compilation of what would be
deemed acceptable by the Council of Nicea etc...}

Connections to the concept of a subterranean
Hell can also be concluded from the writings of the Book of Enoch.

"But now the giants who are born from the
(union of) spirits and the flesh shall be called evil spirits upon
the earth, because their dwelling shall be
upon the earth and inside the earth. Evil spirits have
come out of their bodies. Because from the day that they were
created from the holy ones they became the Watchers; their first
origin is the spiritual foundation. They will become evil upon the
earth and shall be called evil spirits. The dwelling of the
spiritual beings of heaven is heaven; but the dwelling of the
spirits of the earth, which are born upon the earth, is in the
earth."

Jubilees 5:1 And it came to pass when the children of men began to
multiply on the face of the earth and daughters were born unto them,
that the angels of God saw them on a certain year of this jubilee,
that they were beautiful to look upon; and they took themselves
wives of all whom they
2 chose, and they bare unto them sons and they were giants. And
lawlessness increased on the earth and all flesh corrupted its way,
alike men and cattle and beasts and birds and everything that walks
on the earth -all of them corrupted their ways and their orders, and
they began to devour each other, and lawlessness increased on the
earth and every imagination of the thoughts of all men .

In the
The Testaments of the Twelve Patriarchs , the fall of the angels is also mentioned.
A brief reference stating that the Watchers "changed the order of their
nature." (Naphtali 3:5) And also in Reuben, where Reuben accuses women
of seeking to ensnare men.

"Thus they allured the Watchers before the Flood, for as these
continually beheld them, they lusted after them and conceived the
act in their mind; for they changed themselves unto the shape of men
and appeared to them when they were with their husbands; and the
women, lusting in their minds after their forms, gave birth to
giants, for the Watchers appeared to them as reaching up to heaven."

Reuben account differs in that the Women were
physically mating with their husbands and "lusting in their
minds after their forms,... for the Watchers appeared to them as
reaching up to heaven."

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Similarities to The Fallen Angel
saga can be found in the ancient Greek
drama of the Titans vs. (Titan translated means overreacher
which would equate once more to Pride} the Olympians,
and are probably an ancient interpretation of a conflict that took
place between angelic beings, they labeled Gods. Greek
mythology can be traced back to the ancient Middle Eastern cultures
of Mesopotamia, As can the Israelites through Abraham.

In the Greek tale Cronus is the Patriarch
of the Titans. Titans fought with Cronus against Zeus and were
punished by being banished to Tartarus. Tartarus
bears a strong resemblance to the current Concept of Hell. It is interesting to note that Chaos ,
the Creator God is not mentioned in this conflict.

*There
is also a similar account of this battle of the Angels in
Babylonian/Chaldean mythology in the struggle between
Merodach{Marduk} and the dragon Tiamat . Bel-Merodach, was the
champion of the "Gods" of heaven, assailing Tiamat, the power
of the primeval deep.